As electronic devices continue to shrink, space on circuit boards becomes a critical consideration. Shielding takes up a significant portion of board space. Furthermore, the need for complete perimeter ground contact is an electrical necessity to provide sub-circuit isolation.
Traditional shielding approaches have used sheet metal cans soldered onto a board. The problem with the soldered-can approach is that considerable board space is required in the x-y planes, especially when using side by side solder tracks. A process known as pinch trimming can be used to eliminate the small lip around the can prior to the can being soldered to the board. While pinch trimming minimizes the required width of solder tracks, significant board space is still required when side by side cans are used. Traditional cans also take up space in the z-plane due to the sheet metal thickness and additional clearance gap needed between the sheet metal and board.
Several compliant conductive elastomer approaches are available as alternatives to solder cans. Dispensing beads onto a sheet metal can, metalized plastic or casting is one approach. However, dispensed beads have limited z-height causing compressive loads to be high. Another approach is to overmold a conductive elastomer directly over a sheet metal can, metalized plastic or casting. The overmolded can approach combines metal or metalized plastic cans with a conductive gasket overmolded directly to the can. The disadvantage to the overmolded can approach is that it is not z-space efficient particularly in stacked board assemblies in which one board is used to complete shielding via a ground plane.
A spacer gasket approach can be used to minimize z in stacked board assemblies where one board is completing the shield via a ground plane. In the spacer gasket approach a plastic gasket is first molded with a desired compartmentalization. Afterwards, a conductive elastomer is overmolded onto the side walls of each of the compartments in the plastic. Unfortunately, the spacer approach requires injection molding tools for both plastic and elastomer and is thus tooling intensive. Furthermore, because the elastomer is molded onto the sides of the plastic walls, large track widths (2 mm) are required thus making the spacer gasket approach impractical for miniaturized designs having tight board space requirements.
Accordingly, there is a need for an improved shield assembly. The ability to use a narrow width shield track would be particularly beneficial to communications products having tight space requirements.